(a) Field of the Invention
This invention relates to forming fabrics for paper making machines and is particularly directed to the provision of an improved duplex forming fabric comprising two layers of synthetic weft strands woven with synthetic warp strands.
(b) Description of Prior Art
Forming fabrics for paper making machines should provide uniform support for the fibres of the pulp stock so that marking of the formed web of paper by aberrations at the supporting surface will be minimized. The fabrics must be stable in the plane of the cloth, flexible at least in the machine direction, resist stretching, resist wear and at the same time provide sufficient drainage capacity.
For many years, forming fabrics were woven of metal strands and while these "wires", as they are called, provided most of the essential requirements, they had a short life span due to failure of the metal strands to resist flexural fatigue, wear and corrosion. Further, due to the nature of metal strands the woven wires could be easily damaged and damaged areas were generally not repairble.
In recent years, forming wires have been woven of plastic polymeric strands and while these have largely overcome the disadvantages of metal strands insofar as resistance of fatigue, wear, corrosion and inadvertent damage is concerned, some of the more desirable qualities of the metal strands were lost. For example, difficulties have been experienced with plastic fabrics that have been woven in the same manner as metal wires, that is, with about 50% warp fill, with respect to dimensional stability, resistance to stretching and also with respect to drainage and fibre support. Although many improvements have been made to produce reasonably satisfactory synthetic forming fabric, still some of the desirable properties of metal fabrics have not been regained.
Recently, synthetic forming fabrics have been woven in duplex weaves having two or more layers of interwoven weft strands and these have provided greater dimensional stability and resistance to stretching while maintaining the good wearing and damage resistant qualities of single layer synthetic fabric. Duplex fabrics are woven with 100% warp fill or greater and due to the nature of the weave, inevitably have an uneven surface that tends to leave a characteristic and objectionable mark on the surface of the paper. Moreover, no amount of stretching during heat setting will alleviate this objectionable sheet marking condition, but, in fact, will generally make it worse.
"Warp fill" is defined as the amount of warp in a given space relative to the total space considered. For example, 50% warp fill means that 50% of the space in the weft direction is taken up by warp. For example, a 68 mesh fabric (i.e. 68 warp strands per inch of width) having 0.008 inch diameter warp strands would have a warp fill factor of 68.times.0.008.times.100=54.4%. Wrap fill can be over 100% when there are more warp strands jammed into the available space than the space can dimensionally accommodate in a single plane. Fabrics having a nominal warp fill of approximately 100% will generally have an actual calculated warp fill of from 90% to 125%. Values over 100% are brought about by crowding and lateral undulation of the warp strands.
Attempts have been made to produce in the double layer fabric a monoplane surface by interweaving the upper layer of weft strands in such a way that when tension is applied during heat setting the top layer weft strands will behave like those of a single layer fabric and, due to crimp exchange, a more monoplane surface will be produced. Such a fabric is taught in U.S. Pat. Nos. 4,071,050 and 4,041,989 issued to Codorniu and Johansson et al; respectively.
The disadvantage of the fabric of these patents is that like most other double layer forming fabrics of the prior art, all of which have about 100% warp fill, the warp strands are woven so that each one rises to the upper surface to bind the upper layer of weft. Adjacent warp strands contact each other where they cross between the weft strands at the upper layer and this results in restricted drainage due to a lack of mesh opening facing the pulp stock at the places where these adjacent warp strands cross each other.